Synopsis The Bell 206B helicopter was being utilized to sling barrels of fuel at a remote tent camp. As the pilot was about to place two barrels near the camp generator shed, he lost tail rotor authority. The helicopter rotated rapidly to the right through approximately three revolutions, struck the ground upright, and rolled onto the right side. The pilot received minor injuries and the helicopter was substantially damaged. The Board determined that the aft short tail rotor drive shaft failed in an area weakened by intergranular cracking which initiated from corrosive attack on the interior wall of the shaft. The drive shaft failure resulted in a loss of tail rotor thrust. The failure occurred when the helicopter was hovering at an altitude which precluded a successful autorotation. 1.0 Factual Information 1.1 History of the Flight The Bell 206B helicopter was being utilized to sling barrels of fuel from a beach to a nearby tent camp, with barrel clamps and a 20-foot lanyard. After transporting several barrels to the helicopter pad, the pilot slung two barrels to the camp generator. As the helicopter approached the generator shed, the pilot transitioned to a hover and began a vertical descent to set the barrels on the ground. When the barrels were approximately two feet above the ground, the helicopter departed from controlled flight and rotated violently to the right through three rotations. A ground assistant, who had positioned himself under the helicopter to stabilize the barrels and release the barrel clamps, realized the helicopter was out of control and ran toward the generator shed to escape. Immediately thereafter, the helicopter struck the ground and rolled onto the right side. The ground assistant was uninjured. The pilot sustained minor injuries, and the helicopter was substantially damaged. 1 All times are MDT (Coordinated Universal Time [UTC] minus six hours unless otherwise stated. 2 Units are consistent with official manuals, documents, reports, and instructions used by or issued to the crew. 3 See Glossary for all abbreviations and acronyms. The accident occurred at latitude 6332'N and longitude 10807'W, at approximately 1800 mountain daylight saving time (MDT)1, during the hours of daylight, at an elevation of approximately 1,250 feet2 above sea level (asl)3. 1.2 Injuries to Persons 1.3 Damage to Aircraft The helicopter sustained substantial damage due to the impact and roll-over. The skid cross tubes detached from the fuselage, and the main rotor struck and severed the tail boom. The cockpit and cabin remained intact. 1.4 Other Damage No significant property damage occurred. 1.5 Personnel Information Captain Age - 32 Pilot Licence - CPL Medical Expiry Date - 01 March 1995 Total Flying Hours - 2,300 Hours on Type - 2,000 Hours Last 90 Days - 180 Hours on Type Last 90 Days - 180 Hours on Duty Prior to Occurrence - 9 Hours off Duty Prior to Work Period - 16 The pilot was licensed in accordance with existing regulations, and qualified on Bell 206B helicopters. He had flown C-FPQS approximately 60 hours since being assigned to the helicopter on 17 August 1994. He had not experienced any mechanical problem with the helicopter during that time. The pilot did not report any physiological or psychological factors which may have affected his performance. 1.5.1 Pilot Action The pilot was aware that the ground assistant was under the helicopter when the loss of tail rotor thrust occurred. The pilot immediately lifted the collective and attempted to manoeuvre away from the generator shed and the ground assistant. The helicopter travelled approximately 50 feet before impact. The pilot did not release the sling load to avoid the risk of dropping the load on the ground assistant. The pilot did not attempt to enter autorotation due to the altitude, the low airspeed, and the perilous location of the ground assistant. 1.6 Aircraft Information Manufacturer - Bell Helicopter Textron Inc. Type and Model - 206B JetRanger III Year of Manufacture - 1981 Serial Number - 3231 Certificate of Airworthiness (Flight Permit) - Valid Total Airframe Time - 10,407 hr Engine Type (number of) Allison 250-C20B - (1) Propeller/Rotor Type (number of) Semi-Rigid - (1) Maximum Allowable Take-off Weight - 3,200 lb Recommended Fuel Type(s) - Jet A, Jet A-1, Jet B Fuel Type Used - Jet B The helicopter had been imported to Canada in May 1993, and was registered to the operator in June 1993. It had previously been American-registered as N825H, and had been utilized for off- shore oil platform work in the Arabian Gulf for approximately 11 years. The Arabian Gulf is a highly corrosive, salt water operating environment, and the summer days are extremely hot and humid. The Canadian log-books indicated that the helicopter had been maintained in accordance with applicable requirements since import. The weight of the helicopter and sling load was estimated to be 3,150 pounds. The normal category maximum gross weight is 3,200 pounds. The Bell 206B is permitted to operate at 3,350 pounds gross weight for slinging operations. The centre of gravity was within the prescribed limits. 1.7 Meteorological Information Good visual meteorological conditions existed at the time of the occurrence, and weather was not considered to be a factor. The pilot and ground assistant reported that the sky was clear with unlimited visibility. The winds were reported as light and variable, and the temperature was estimated to be 5 to 10 degrees Celsius. 1.8 The Bell 206B Tail Rotor Drive System The Bell 206B JetRanger III tail rotor drive system consists of a forward short shaft, an oil cooler fan drive shaft, an aft short shaft, and five aft tail rotor drive shaft segments. The forward short shaft and the oil cooling fan drive shaft are steel. The aft short shaft and the tail rotor drive shaft segments are aluminum. Steel-laminated flexible couplings are used to connect the shaft sections. Bell 206B helicopters serial numbered 4004 and earlier, such as the accident helicopter, are fitted with a hollow aft short tail rotor drive shaft which is open at both ends. Bell 206B helicopters serial numbered 4005 and subsequent are fitted with a 206-040-369-1 drive shaft which is sightly larger in diameter, and is plugged at both ends. Figure 1 - Bell 206 Power Train System 1.9 Wreckage and Impact Information The accident occurred on open, flat tundra. The ground surface was shallow, grass-covered hummocks. Examination of the wreckage by the operator revealed that the tail rotor drive train aft of the oil cooler blower assembly was discontinuous, and that the oil cooler blower impeller had sustained extensive separation from the vane. The tail rotor drive shaft assembly, the oil cooler impeller assembly, the tail rotor gear box, and the tail rotor were forwarded to the TSB Regional Wreckage Facility for examination. Visual examination confirmed that the aluminum aft short tail rotor drive shaft (Part No. 206- 040-330-003) had failed in the vicinity of the forward bonded coupling. An approximately one- inch-long, irregular-shaped section of the shaft was missing between the forward and aft fracture surfaces, and was not recovered. The tail rotor gear box components, the laminated flexible couplings, and the tail rotor blades displayed no evidence of a pre-occurrence tail rotor strike. 1.10 The History of the Aft Short Tail Rotor Drive Shaft The Bell 206B aft short tail rotor drive shaft is not a serialized item, and there is no requirement to maintain an applicable component history card. Seven boxes of maintenance records, which had accompanied the helicopter at import, were reviewed to determine the history of the drive shaft. Relevant documents indicated that the drive shaft had been replaced in February of 1988 at 6,262:10 airframe hours, and that the replacement drive shaft had been removed in serviceable condition from another helicopter, registered as N719H. Consultation with the company which previously owned both helicopters determined that N719H was an older Bell 206 which had been retired from service due to voids in the roof shell assembly. The retired helicopter had accumulated 15,006:10 airframe hours at the time it was withdrawn from service, and no evidence was found to indicate that the original aft short tail rotor drive shaft had ever been replaced. N719H had been operated in the Arabian Gulf for approximately 10 years, and had been operated in the Gulf of Mexico prior to that. The Gulf of Mexico is also considered a highly corrosive, salt water operating environment. 1.11 TSB Engineering Branch Laboratory Examination The aft short tail rotor drive shaft and the oil cooler impeller assembly were submitted to the TSB Engineering Branch Laboratory for detailed examination. The operator voluntarily submitted a second Bell 206B aft short drive shaft for use as a comparison. The exemplar drive shaft had been removed from a company sister ship due to external mechanical damage. The sister ship, C-FPRB, was one serial number subsequent to C-FPQS, and had a nearly identical time-in-service and operating history in the Arabian Gulf. No corrosion was visible on the exterior of the failed drive shaft. However, the external wall had been reworked at some time in the past. The anodizing had been removed and the abraded metal surface had been coated with clear lacquer. Alodine chemical film material had not been applied to the exterior wall, as required, following removal of the original protective coating. The part number, vendor identification, and date of manufacture, which are normally present as inked markings on the tube surface, were not visible. The failed drive shaft was manufactured from 2024 T3 aluminum and had a wall thickness of .060 inches, as specified in the engineering drawing for the Bell Part No. 206-040-330 assembly. The interior tube wall exhibited a number of fine scratches which had penetrated the anodized and lacquered surface, and several areas of corrosion. Scanning electron microscope examination of the forward and aft fracture surfaces identified an area of environmentally assisted cracking (EAC) and an area of final overload fracture, indicating two distinct failure modes. The EAC fracture zone occupied greater than 35 per cent of the cross sectional area of the forward end fracture. The fracture surfaces displayed wholly intergranular features accompanied by corrosion product deposits. The tube wall showed evidence of exfoliation action. Analysis of the corrosion product deposits disclosed the presence of elements such as sodium, sulphur, chlorine, potassium, and calcium, which are the major constituents of sea water. The exemplar drive shaft assembly showed a single incomplete inked marking (....9?23-80-1) on the shaft surface, and an inked date marking (9-19-80) on both the fore and aft attachment flanges. The original grey-coloured surface protection finish on the shaft and adaptor ends was intact, although only small patches of the lacquer coating remained. The exemplar shaft did not appear to have been reworked, and there was no evidence of corrosion on the outer or inner walls. 1.12 Aft Short Tail Rotor Drive Shaft Inspection Requirements The Bell 206B JetRanger III Component Repair and Overhaul Manual provides the overhaul and repair instructions applicable to the tail rotor drive shaft system. It also identifies the mechanical and corrosion damage limits for the component. The damage limits pertain to externally visible defects, and there is no stated procedure to examine the bore of the drive shaft for corrosion. The Bell 206B III Maintenance Manual identifies the scheduled inspection requirements for the helicopter. The 100-hour inspection tasks include a requirement to check the segmented drive shaft for condition and security. However, there is no specific reference to the aft short tail rotor drive shaft, nor is scheduled non-destructive testing (NDT) or overhaul of the component required. 1.13 The Service Difficulty Reporting Data Base The Transport Canada Service Difficulty Data Base was searched to locate similar reports of Bell 206B aft short tail rotor drive shaft discrepancies and failures. There was no record of drive shaft replacement or failure due to internal corrosion. 1.14 The Oil Cooler Impeller Fan Examination of the oil cooler impeller determined that all 36 vanes had separated from the forward end plate, while a small number had remained partially attached to the aft end plate of the impeller assembly. Examination of the surfaces of the vanes failed to disclose any evidence of massive impact with foreign or ingested objects. The fracture surfaces were found to show ductile overload features, with no evidence of a progressive failure. Although the precise cause of failure was not determined, the destruction of the oil cooler impeller was considered secondary to the aft short tail rotor drive shaft failure. 1.15 Tail Rotor Thrust Failure The Bell 206B JetRanger III Flight Manual states that, in the event of a complete loss of tail rotor thrust, the pilot is to Reduce throttle to flight idle, immediately enter autorotation and maintain a minimum airspeed of 58 MPH IAS (50 knots) during descent. The helicopter was hovering at approximately 25 feet above ground when the loss of tail rotor thrust occurred. The height/velocity diagram in the Bell 206B JetRanger III Flight Manual identifies the airspeed versus altitude limitations for safe autorotation landings. The helicopter was operating within the designated avoid operation area when the failure occurred, which indicates that an autorotational landing without damage would be unlikely. Standard operating procedure dictates that an external sling load be released in the event of an in-flight emergency. 1.16 Survival Aspects The fuselage structure remained rigid during the accident, and the occupiable cockpit survival space was not compromised. The pilot sustained a bruised right leg. He was not wearing the available shoulder harness; he was, however, wearing a protective helmet. Scars on the outside of the shell indicated that the helmet had effectively prevented serious head injury. The pilot was able to shut down the engine, fuel boost pumps, and electrical system, and evacuate the helicopter quickly without assistance. There was no evidence of a post-impact fire. A bystander observed smoke rising from the engine exhaust stack, and immediately discharged a fire extinguisher into the engine compartment as a precaution.